Many plastics tend to degrade when exposed to sunlight or other sources of ultraviolet (UV) light. This exposure results in, for example, loss of tensile strength, embrittlement, and discoloration. To help prevent such degradation, ultraviolet light stabilizers are often incorporated into a polymer composition, which is used as a protective top layer for underlying materials. These uv light stabilizers perform well but are often not stable in or compatible with the polymer, which results in the stabilizer leaving the polymer by leaching, migration, vaporization, or other means. When the uv stabilizer levels are lowered in this manner, their protective effect is diminished, leading to degradation of the polymer and the underlying materials.
Molecules that function as ultraviolet light absorbers are generally known in the art. For example, in U.S. Pat. No. 4,853,471 Rody et al. describe 2-(2-hydroxyphenyl)-benzotriazole compounds that are useful as ultraviolet light absorbers. These compounds are described as useful in the stabilization of a wide variety of polymeric materials against ultraviolet light, particularly those that are used in the preparation of photographic materials. Other classes of ultraviolet light absorbers include the 2-hydroxybenzophenones and the diphenylcyanoacrylates.
While such compounds are known to absorb ultraviolet light, their incompatibility with the polyolefins often used as the top film in multilayer sheetings has limited their usefulness in such constructions. The low solubility and high mobility of typical ultraviolet light absorbers in polyolefins results in an inability to maintain high levels of these compounds in polyolefin films; the compounds tend to bloom or migrate out of the film, and the protective properties of the compounds are lost.
Much effort has been directed to the development of methods of maintaining the uv stabilizer compound in a selected polymeric matrix. Ultraviolet light stabilizers have been developed that have polymerizable functionalities. The polymerizable stabilizer is then incorporated into the polymer as a co-monomer or copolymerized with another monomer to form a stabilizing polymer that can be added to the desired polymer. Because the stabilizing compound is then part of the polymer backbone, it remains in the polymer composition and the stabilizing benefits are retained for a longer period of time. Examples of incorporation of a stabilizing monomer into a polymer backbone may be found in, for example, Vogl et al., POLYMER, Vol. 26, p. 1288 (1985); U.S. Pat. No. 3,313,866 (Horton et al.), U.S. Pat. No. 4,128,536 (Brodsky et al.), and U.S. Pat. No. 4,528,311 (Beard et al.).
There has also been development in the area of "attaching" the ultraviolet light absorbing compound to a polymer backbone by selecting UV light absorbers with groups that react with various functionalities present on the polymer. Fu et al., in U.S. Pat. No. 4,413,096, describe .alpha.-olefin copolymers wherein hindered amine light stabilizers are pendant to the polymer backbone. The Fu copolymers may be prepared by transesterification of the polymer with the hindered amine light stabilizing compound.
There is a continuing need in the art for a method of stabilizing polymers such as polyolefins with ultraviolet light absorbing compounds that provides lasting protection both to the polyolefin and to articles that incorporate the polyolefin.